NAME

SYNOPSIS

DESCRIPTION

Devel::Peek contains functions which allows raw Perl data-
types to be manipulated from a Perl script. This is used by
those who do XS programming to check that the data they are
sending from C to Perl looks as they think it should look.
The trick, then, is to know what the raw datatype is sup-
posed to look like when it gets to Perl. This document
offers some tips and hints to describe good and bad raw
data.
It is very possible that this document will fall far short
of being useful to the casual reader. The reader is
expected to understand the material in the first few sec-
tions of perlguts.
Devel::Peek supplies a "Dump()" function which can dump a
raw Perl datatype, and "mstat("marker")" function to report
on memory usage (if perl is compiled with corresponding
option). The function DeadCode() provides statistics on the
data "frozen" into inactive "CV". Devel::Peek also supplies
"SvREFCNT()", "SvREFCNT_inc()", and "SvREFCNT_dec()" which
can query, increment, and decrement reference counts on SVs.
This document will take a passive, and safe, approach to
data debugging and for that it will describe only the
"Dump()" function.
Function "DumpArray()" allows dumping of multiple values
(useful when you need to analyze returns of functions).
The global variable $Devel::Peek::pv_limit can be set to
limit the number of character printed in various string
values. Setting it to 0 means no limit.
If "use Devel::Peek" directive has a ":opd=FLAGS" argument,
this switches on debugging of opcode dispatch. "FLAGS"
should be a combination of "s", "t", and "P" (see -D flags
in perlrun). ":opd" is a shortcut for ":opd=st".
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Runtime debugging
"CvGV($cv)" return one of the globs associated to a subrou-
tine reference $cv.
debug_flags() returns a string representation of $^D (simi-
lar to what is allowed for -D flag). When called with a
numeric argument, sets $^D to the corresponding value. When
called with an argument of the form "flags-flags", set
on/off bits of $^D corresponding to letters before/after
"-". (The returned value is for $^D before the modifica-
tion.)
runops_debug() returns true if the current opcode dispatcher
is the debugging one. When called with an argument,
switches to debugging or non-debugging dispatcher depending
on the argument (active for newly-entered subs/etc only).
(The returned value is for the dispatcher before the modifi-
cation.)
Memory footprint debugging
When perl is compiled with support for memory footprint
debugging (default with Perl's malloc()), Devel::Peek pro-
vides an access to this API.
Use mstat() function to emit a memory state statistic to the
terminal. For more information on the format of output of
mstat() see "Using $ENV{PERL_DEBUG_MSTATS}" in perldebguts.
Three additional functions allow access to this statistic
from Perl. First, use "mstats_fillhash(%hash)" to get the
information contained in the output of mstat() into %hash.
The field of this hash are
minbucket nbuckets sbrk_good sbrk_slack sbrked_remains sbrks start_slack
topbucket topbucket_ev topbucket_odd total total_chain total_sbrk totfree
Two additional fields "free", "used" contain array refer-
ences which provide per-bucket count of free and used
chunks. Two other fields "mem_size", "available_size" con-
tain array references which provide the information about
the allocated size and usable size of chunks in each bucket.
Again, see "Using $ENV{PERL_DEBUG_MSTATS}" in perldebguts
for details.
Keep in mind that only the first several "odd-numbered"
buckets are used, so the information on size of the
"odd-numbered" buckets which are not used is probably mean-
ingless.
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The information in
mem_size available_size minbucket nbuckets
is the property of a particular build of perl, and does not
depend on the current process. If you do not provide the
optional argument to the functions mstats_fillhash(),
fill_mstats(), mstats2hash(), then the information in fields
"mem_size", "available_size" is not updated.
"fill_mstats($buf)" is a much cheaper call (both speedwise
and memory-wise) which collects the statistic into $buf in
machine-readable form. At a later moment you may need to
call "mstats2hash($buf, %hash)" to use this information to
fill %hash.
All three APIs "fill_mstats($buf)",
"mstats_fillhash(%hash)", and "mstats2hash($buf, %hash)" are
designed to allocate no memory if used the second time on
the same $buf and/or %hash.
So, if you want to collect memory info in a cycle, you may
call
$#buf = 999;
fill_mstats($_) for @buf;
mstats_fillhash(%report, 1); # Static info too
foreach (@buf) {
# Do something...
fill_mstats $_; # Collect statistic
}
foreach (@buf) {
mstats2hash($_, %report); # Preserve static info
# Do something with %report
}

EXAMPLES

The following examples don't attempt to show everything as
that would be a monumental task, and, frankly, we don't want
this manpage to be an internals document for Perl. The
examples do demonstrate some basics of the raw Perl data-
types, and should suffice to get most determined people on
their way. There are no guidewires or safety nets, nor
blazed trails, so be prepared to travel alone from this
point and on and, if at all possible, don't fall into the
quicksand (it's bad for business).
Oh, one final bit of advice: take perlguts with you. When
you return we expect to see it well-thumbed.
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A simple scalar string
Let's begin by looking a simple scalar which is holding a
string.
use Devel::Peek;
$a = "hello";
Dump $a;
The output:
SV = PVIV(0xbc288)
REFCNT = 1
FLAGS = (POK,pPOK)
IV = 0
PV = 0xb2048 "hello"\0
CUR = 5
LEN = 6
This says $a is an SV, a scalar. The scalar is a PVIV, a
string. Its reference count is 1. It has the "POK" flag
set, meaning its current PV field is valid. Because POK is
set we look at the PV item to see what is in the scalar.
The \0 at the end indicate that this PV is properly
NUL-terminated. If the FLAGS had been IOK we would look at
the IV item. CUR indicates the number of characters in the
PV. LEN indicates the number of bytes requested for the PV
(one more than CUR, in this case, because LEN includes an
extra byte for the end-of-string marker).
A simple scalar number
If the scalar contains a number the raw SV will be leaner.
use Devel::Peek;
$a = 42;
Dump $a;
The output:
SV = IV(0xbc818)
REFCNT = 1
FLAGS = (IOK,pIOK)
IV = 42
This says $a is an SV, a scalar. The scalar is an IV, a
number. Its reference count is 1. It has the "IOK" flag
set, meaning it is currently being evaluated as a number.
Because IOK is set we look at the IV item to see what is in
the scalar.
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A simple scalar with an extra reference
If the scalar from the previous example had an extra refer-
ence:
use Devel::Peek;
$a = 42;
$b = \$a;
Dump $a;
The output:
SV = IV(0xbe860)
REFCNT = 2
FLAGS = (IOK,pIOK)
IV = 42
Notice that this example differs from the previous example
only in its reference count. Compare this to the next exam-
ple, where we dump $b instead of $a.
A reference to a simple scalar
This shows what a reference looks like when it references a
simple scalar.
use Devel::Peek;
$a = 42;
$b = \$a;
Dump $b;
The output:
SV = RV(0xf041c)
REFCNT = 1
FLAGS = (ROK)
RV = 0xbab08
SV = IV(0xbe860)
REFCNT = 2
FLAGS = (IOK,pIOK)
IV = 42
Starting from the top, this says $b is an SV. The scalar is
an RV, a reference. It has the "ROK" flag set, meaning it
is a reference. Because ROK is set we have an RV item
rather than an IV or PV. Notice that Dump follows the
reference and shows us what $b was referencing. We see the
same $a that we found in the previous example.
Note that the value of "RV" coincides with the numbers we
see when we stringify $b. The addresses inside RV() and IV()
are addresses of "X***" structure which holds the current
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state of an "SV". This address may change during lifetime of
an SV.
A reference to an array
This shows what a reference to an array looks like.
use Devel::Peek;
$a = [42];
Dump $a;
The output:
SV = RV(0xf041c)
REFCNT = 1
FLAGS = (ROK)
RV = 0xb2850
SV = PVAV(0xbd448)
REFCNT = 1
FLAGS = ()
IV = 0
NV = 0
ARRAY = 0xb2048
ALLOC = 0xb2048
FILL = 0
MAX = 0
ARYLEN = 0x0
FLAGS = (REAL)
Elt No. 0 0xb5658
SV = IV(0xbe860)
REFCNT = 1
FLAGS = (IOK,pIOK)
IV = 42
This says $a is an SV and that it is an RV. That RV points
to another SV which is a PVAV, an array. The array has one
element, element zero, which is another SV. The field "FILL"
above indicates the last element in the array, similar to
"$#$a".
If $a pointed to an array of two elements then we would see
the following.
use Devel::Peek 'Dump';
$a = [42,24];
Dump $a;
The output:
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SV = RV(0xf041c)
REFCNT = 1
FLAGS = (ROK)
RV = 0xb2850
SV = PVAV(0xbd448)
REFCNT = 1
FLAGS = ()
IV = 0
NV = 0
ARRAY = 0xb2048
ALLOC = 0xb2048
FILL = 0
MAX = 0
ARYLEN = 0x0
FLAGS = (REAL)
Elt No. 0 0xb5658
SV = IV(0xbe860)
REFCNT = 1
FLAGS = (IOK,pIOK)
IV = 42
Elt No. 1 0xb5680
SV = IV(0xbe818)
REFCNT = 1
FLAGS = (IOK,pIOK)
IV = 24
Note that "Dump" will not report all the elements in the
array, only several first (depending on how deep it already
went into the report tree).
A reference to a hash
The following shows the raw form of a reference to a hash.
use Devel::Peek;
$a = {hello=>42};
Dump $a;
The output:
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SV = RV(0x8177858) at 0x816a618
REFCNT = 1
FLAGS = (ROK)
RV = 0x814fc10
SV = PVHV(0x8167768) at 0x814fc10
REFCNT = 1
FLAGS = (SHAREKEYS)
IV = 1
NV = 0
ARRAY = 0x816c5b8 (0:7, 1:1)
hash quality = 100.0%
KEYS = 1
FILL = 1
MAX = 7
RITER = -1
EITER = 0x0
Elt "hello" HASH = 0xc8fd181b
SV = IV(0x816c030) at 0x814fcf4
REFCNT = 1
FLAGS = (IOK,pIOK)
IV = 42
This shows $a is a reference pointing to an SV. That SV is
a PVHV, a hash. Fields RITER and EITER are used by "each".
The "quality" of a hash is defined as the total number of
comparisons needed to access every element once, relative to
the expected number needed for a random hash. The value can
go over 100%.
The total number of comparisons is equal to the sum of the
squares of the number of entries in each bucket. For a ran-
dom hash of "<n"> keys into "<k"> buckets, the expected
value is:
n + n(n-1)/2k
Dumping a large array or hash
The "Dump()" function, by default, dumps up to 4 elements
from a toplevel array or hash. This number can be increased
by supplying a second argument to the function.
use Devel::Peek;
$a = [10,11,12,13,14];
Dump $a;
Notice that "Dump()" prints only elements 10 through 13 in
the above code. The following code will print all of the
elements.
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use Devel::Peek 'Dump';
$a = [10,11,12,13,14];
Dump $a, 5;
A reference to an SV which holds a C pointer
This is what you really need to know as an XS programmer, of
course. When an XSUB returns a pointer to a C structure
that pointer is stored in an SV and a reference to that SV
is placed on the XSUB stack. So the output from an XSUB
which uses something like the T_PTROBJ map might look some-
thing like this:
SV = RV(0xf381c)
REFCNT = 1
FLAGS = (ROK)
RV = 0xb8ad8
SV = PVMG(0xbb3c8)
REFCNT = 1
FLAGS = (OBJECT,IOK,pIOK)
IV = 729160
NV = 0
PV = 0
STASH = 0xc1d10 "CookBookB::Opaque"
This shows that we have an SV which is an RV. That RV
points at another SV. In this case that second SV is a
PVMG, a blessed scalar. Because it is blessed it has the
"OBJECT" flag set. Note that an SV which holds a C pointer
also has the "IOK" flag set. The "STASH" is set to the
package name which this SV was blessed into.
The output from an XSUB which uses something like the
T_PTRREF map, which doesn't bless the object, might look
something like this:
SV = RV(0xf381c)
REFCNT = 1
FLAGS = (ROK)
RV = 0xb8ad8
SV = PVMG(0xbb3c8)
REFCNT = 1
FLAGS = (IOK,pIOK)
IV = 729160
NV = 0
PV = 0
A reference to a subroutine
Looks like this:
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SV = RV(0x798ec)
REFCNT = 1
FLAGS = (TEMP,ROK)
RV = 0x1d453c
SV = PVCV(0x1c768c)
REFCNT = 2
FLAGS = ()
IV = 0
NV = 0
COMP_STASH = 0x31068 "main"
START = 0xb20e0
ROOT = 0xbece0
XSUB = 0x0
XSUBANY = 0
GVGV::GV = 0x1d44e8 "MY" :: "top_targets"
FILE = "(eval 5)"
DEPTH = 0
PADLIST = 0x1c9338
This shows that
+ the subroutine is not an XSUB (since "START" and "ROOT"
are non-zero, and "XSUB" is zero);
+ that it was compiled in the package "main";
+ under the name "MY::top_targets";
+ inside a 5th eval in the program;
+ it is not currently executed (see "DEPTH");
+ it has no prototype ("PROTOTYPE" field is missing).

EXPORTS

BUGS

Readers have been known to skip important parts of perlguts,
causing much frustration for all.

AUTHOR

Ilya Zakharevich ilya@math.ohio-state.edu
Copyright (c) 1995-98 Ilya Zakharevich. All rights reserved.
This program is free software; you can redistribute it
and/or modify it under the same terms as Perl itself.
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Author of this software makes no claim whatsoever about sui-
tability, reliability, edability, editability or usability
of this product, and should not be kept liable for any dam-
age resulting from the use of it. If you can use it, you are
in luck, if not, I should not be kept responsible. Keep a
handy copy of your backup tape at hand.